Process for improving the machining qualities of steel



United States Patent PROCESS FOR'IMPROVING THE MACHINING QUALITIES OF STEEL No Drawing. Application April 6, 1953 Serial No. 347,170

8 Claims. c1. 14s 21.s

My process relates to the treatment of steels for the purpose of improving their machinability, more particularly with respect to increasing tool life and increasing the production'by the use of higher feeds and speeds.

My invention comprises the steps of rapidly cooling steel from a temperature somewhat below the lower critical point of the steel to a temperature at least as low as 550 F. and then maintaining the steel at a temperature etween 300 F. and 550 F. for at least one hour.

In setting forth my invention I shall first give a specific example and then indicate how various factors may be changed and still obtain'the essential results of my invention.

In my specific example I shall describe the treatment of a steel of the following composition:

C .10 Mn .85 P .095 S .30 N .012 Si .01 max.

This steel has a lower critical point of approximately 1300" F. The steel is allowed to cool from the mill heat to a temperature of 1100 F. and then is immersed in Water to rapidly cool it until it has reached a temperature of 400 F.; it is then put into a furnace maintained at a temperature of 400 F. and held at this temperature for 8 hours. The steel is then allowed to cool to atmospheric temperatures, after which the steel was cold drawn.

When this steel, thus treated, is machined the tool life is increased from 50% to 150% over the tool life when the same steel, but untreated, is machined. By this treatment the production is increased by about 25%, that is to say, the feeds and speeds may be increased sufiiciently to do twenty-five percent (25%) more work than could be done on the same steel when untreated. The improved production or increased tool life is secured without alfecting the finish.

In the example of my invention just given, a specific composition of steel has been given. The process, however, is applicable to a wide range of compositions. The carbon content may vary from 0.05% to 0.40% although it is preferable to use a carbon content not in excess of about 0.30%. The manganese content may vary from 0.30% to 1.70% and the sulphur content from 0.02% to 0.50%. The nitrogen content may vary from 0.002% to 0.04%. The silicon should not be in excess of 0.30%. In general it may be said that, while certain compositions are better than others in machining characteristics, the tool life and production of any steel are increased by the treatment here set forth so long as the carbon is not in excess of 0.40%. The process is equally applicable to open hearth and bessemer steels.

My process is applicable to normal constructional alloy steels containing nickel, copper, chromium or molybdenum or combinations of two or more of these elements,

in which the total of such alloying constituents does not exceed 5 or 6%. In such alloy steels the carbon should not exceed 0.25%.

In the specific example of my process given above the steel is cooled in a normal way from the mill heat until it reaches a temperature of 1100 F. and it is then quenched in water. The temperature of the steel at which the rapid cooling is started must be below the lower critical point of the particular steel being treated. In general, I start the rapid cooling when the steel is at a temperature of from about 40 F. below the lower critical point to about 350 F. below the lower critical point. The temperature of the steel at the time at which the rapid cooling is started should not be below 900 F, It will of course be understood that the lower critical point will vary with different steels. It is not essential to the success of my process that the treatment be carried out just after leaving the mill. It is unimportant, so far as results are concerned, whether the process is so carried out or is carried out upon reheated steel. When a reheated steel is treated by my process it is advisable to reheat it to a temperature above the upper critical point of the particular steel being treated and then allow it to cool normally in air to a temperature below the'lower critical point before rapidly cooling the steel.

In the specific example of my process given above, the steel is quenched to a temperature of 400 F. and then put into the furnace for holding at a temperature of 400 F. It is not necessary that the end of the quench be at the specific temperature mentioned. The rapid cooling should be continued until the temperature of the steel is at least as low as 550 F. I prefer to rapidly cool to a temperature lower than 550 F. The steel may. be rapidly cooled to any temperature below 550 F., even to atmospheric temperatures or lower. Most frequently I rapidly cool to a temperature of from 300 F. to 500 F In the specific example of my process, given above, the steel, after the quenching, is put in a furnace where it is maintained at 400 F. for 8 hours. The process is not limited to this temperature and time. The temperature of the holding operation should not be above 550 F. or less than 300 F. I prefer to maintain the holding temperature of the steel at from 400 F. to 500 F. The time of holding the steel at the desired. temperature should not be less than one hour and preferably not less than two hours. Usually I hold the steel at the desired temperature from four to eight hours.

The rapid cooling step of the specific example given above was by quenching in Water. The cooling must be more rapid than normal cooling in air. The cooling conditions must be such that the temperature drop averages at least per minute during the rapid cooling. Preferably, I employ cooling rates greater than this. In most cases, I use a water quench, as, for example, by immersion in water or by a water spray. If the steel article being treated is not too large the rapid cooling may 'be satisfactorily effected by means of an air spray.

In the specific example, given above, the steel was cold drawn following my process. Cold drawing is not essential to the success of my process in improving machinability but cold drawing is frequently desirable.

I claim:

1. A process for improving the machining qualities of steel containing between .05 and .25% carbon and containing not over 5.00% of alloying ingredients of the group consisting of nickel, copper, chromium, and molybdenum, and the balance consisting essentially of iron, which comprises the steps of rapidly cooling the steel from a temperature below the lower critical point of the particular steel being treated but not less than 900 F.

to "a temperature at least as low as 550 F., and maintaining the steel at a temperature between 300 F. and 550 for at least one hour.

, 2 Aprocessir'irnprovingthe'machining'qualities of "steel containing between .05% and .25% carbon and containing not over 5.00% of alloying ingredients of the gr'bupeons'istiii'g'ofnickel, copper, chromium, and molybdenum, 'iandfthefbalan'ce consisting essentially of iron, 'whichcomprises'the steps of rapidly'cooling the steel from a temperature *of from 40 to 350 F. below the lower critical point 'of the particular steel being treated to a "temperature'not over 550 F., and maintaining the steel at a temperature between 300 F. and 550 F. for at least 'twohours.

, 3. A'processifor'improving the machining qualities of steel'eontaining'btween .05 and .25 carbon and containing 'not'over 5.00% of alloying ingredients of the lgroupbohsis'tingbf nickel, copper, chromium, and'rnolybdenum, and the balance consisting essentially of iron, which comprises the steps of rapidly cooling the steel from a" ferripe'ratu're of from 40 to 350 F. below the lowereritical point or the particular steel being treated to {temperature "not over 550 F., and maintaining the steel'at aten'iperature between 300 F. and 550 F. for from four to eight hours.

4. A process for'irnpfoving the machining qualities of steel 'co'ntainingbe'tw'eeh .05 and .25% carbon and containing not over 5.00% of'alloying ingredients of the group consisting of nickel, copper, chromium, and molybdenum, and the balance consisting essentially of iron, "whi'chcoinprises the steps of rapidly cooling the steel frofn'a temperature of from 40 F. to 350 F. below the iowfe'rfc ritic al point of the particular steel being treated iintil the temperature is'from 300 F. to 500 F., and maintaining "such rapidly cooled steel at a temperature of from 400 F. to 500 F. 'for from two to eight hours.

5. A'proc'ess for improving the machining qualities of steel containing between .05% and .25% carbon and containing not over 5.00% of alloying ingredients of the group consisting of nickel, copper, chromium, and molybdenum,:ma the balanceconsisting essentially of iron, which comprises the steps of quenching the steel from a temperature'below the lower critical point of the particular steel'being treated but not less than 900 F. until the steelis at a temperature at least as low as 500 F., and maintaining such rapidly cooled steel at a temperature of from 300 F. to 500 F. for at least two hours.

'6. A process for improving the machining qualities of steel containing carbon-iron: .05% to 30%, manganese from 30% to 1.70%, sulphur from .02% to nitrogen from 0.002% to 0.04% and silicon not in excess of 30%, containing not over 5.00% of alloying ingredients of the group consisting of nickel, copper, chromium and molybdenum, the carbon content of the steel being not over 0.25% when such alloying ingredients arepresent, which comprises thesteps of quenching the steel from a temperature of from 40 F. to 350 F. below the lower critical point'of the steel being treated'until the steel has a temperature'of from 300 F. to 500 F., and maintaining such'rapidly cooled steel at a temperature of from 300 F. to 500 F. 'for at least two hours.

7. A process for improving the machining qualities of steel containing between .05% and '.25% carbon and containing not over 5.00% of alloying ingredients of the group consisting of nickel, copper, chromium, and molybdenum, and the balance consisting "essentially of iron, which comprises 'the st'eps'of'eooling the steel normally from a temperature above the upper critical point to a temperature below the lower critical point but not less than 900 F., and then cooling rapidly to a temperature at least as low 'as'550" R, andrnlaintaining'the'steel at a temperature'between 300 F. and 550 F. for at least one hour.

8. A process for improving the machining qualities of steel'containing hem/eon fa'nd .25 carbon and containing notover 5.00% of alloying ingredients 'of the group consisting" of nickel, copper, chromium, and molybdenum, and the balance consisting essentially of iron, which comprisesthestepsof cooling the steel normally from a temperatureahovethe upper critical point to a temperature at least 40 F. below the lower criticalpoint but not more than 350 F. below the lower criticalpoint of the particular steelbeing treated, and then rapidly cooling the steel until. the temperature thereof is from 300 F. to 500 F., and maintaining such rapidly cooled steel at a temperature of from 300 F. to 500 F. for from two to eight hours.

References Cited in'the file ofthis'p'atent Steel and Its Heat Treatmen, Bullens, vol. I, fifth edition, 1948, pp. 253-255. Copy in Div. 3.

Transactions, American Society for Metals, vol. 22, 1934,1313. 934. Copy in' Div. 3.

Metals Handbook, 193 9 ed., pages 602-605.

Journal of the "Ir on and Steel Institute, vol. CXVi, No. 2, 1927, pages 293295. Copy in Div. 3. 

1. A PROCESS FOR IMPROVING THE MACHINING QUALITIES OF STEEL CONTAINING BETWEEN .05% AND .25% CARBON AND CONTAINING NOT OVER 5.00% OF ALLOYING INGREDIENTS OF THE GROUP CONSISTING OF NICKEL, COPPER, CHROMIUM, AND MOLYBDENUM, AND THE BALANCE CONSISTIONG ESSENTIALLY OF IRON, WHICH COMPRISES THE STEPS OF RAPIDLY COOLING THE STEEL FROM A TEMPERATURE BELOW THE LOWER CRITICAL POINT OF THE PARTICULAR STEEL BEING TREATED BUT NOT LESS THAN 900*F. TO A TEMPERATURE AT LEAST AS LOW AS 550*F., AND MAINTAINING THE STEEL AT A TEMPERATURE BETWEEN 300*F. AND 550*F. FOR AT LEAST ONE HOUR. 